Recently a disposable test device was developed for the potentiometric analysis of blood serum using two identical ion-selective electrodes (hereinafter, "ISE's"). Examples of such devices are disclosed in U.S. Pat. No. 4,053,381, issued Oct. 11, 1977. The ISE's of such a device are overlaid or otherwise contacted with an ion bridge that provides for ionic flow between a drop of a test liquid and a drop of a reference liquid, the bridge also having apertures allowing the drops to contact their respective electrodes. When an electrometer or potentiometer makes contact with both electrodes, a differential potential is detected that is proportional to the difference in activity, and therefore concentration, of the ion under analysis. An embodiment useful for absolute measurements is also disclosed.
Such a device has permitted rapid, automatic measurements of electrolytes using very small samples of serum. Although such advantages have constituted marked improvements, there have remained certain aspects that are subject to further improvement. In one form of the device, the bridge between the electrodes has comprised an absorbent matrix open to the atmosphere, causing evaporation and loss of, or contamination by, analytes such as CO.sub.2. In another form, the ion bridge has comprised a fibrous matrix the fibers of which are not always restricted to desired orientations. Occasionally such fibers can extend out from the bridge and contact one or more underlayers of the electrode. When a portion of the liquid to be tested wets such fibers, the ISE can become shorted.
Yet another drawback with certain materials used as an ion bridge has been their inability to transport whole blood. For example, a fibrous matrix such as most paper materials has capillary passageways which are too tortuous for the transport of whole blood without hemolysis. The alternative of course is to phase-separate the serum, a time-consuming additional step.
Therefore, prior to this invention there has been an unsatisfied need for an ion bridge that does not utilize fibrous materials as the primary transport media but does allow for the transport of whole blood.